Parallel assembly of amplifiers



J. MERHAUT PARALLEL ASSEMBLY OF AMPLIFIERS Jan. 2, 1951 2 SheeFs-Shet 1 Filed July 15, 1947 l N V E NTOR "yumdgmwu LmLQL Jan. 2, 1951 J. MERHAUT PARALLEL ASSEMBLY OF AMPLIFIERS 2 Sheets-Sheet 2 Filed July 15, 1947 5 rv p 3 zLmm INVENTOR Patented Jan. 2, 1951 FEECE PARALLEL ASSEMBLY OF AMPLIFIERS Josef Merhaut, Prague, Czechoslovakia, assignor to Tesla. slabo roud a radiotechnick zavody, Narodni Podnik, Prague, Czechoslovakia Application July 15, 1947, Serial No. 761,090 In Czechoslovakia August 15, 1946 8 Claims.

This invention relates to amplifying systems and, more particularly, to such systems employing multiple amplifiers. A public address system such as employed by factories, municipalities, railway stations, etc. requires large output powers which may be supplied either by one large output amplifier or else by several smalller units in parallel connection.

From the point of View of rationalisation of production it is convenient to use the second alternative, which makes it possible to employ mass production methods for the manufacture of large quantities of radio valves and amplifiers of a uniform type. The working and maintenance of smaller parallel units is also more advantageous, because the arrangement continues to work even if some of the last stages become defective. It is also necessary to keep a few spare amplifier units to replace defective ones, and the assembly may be enlarged by the addition of more units.

The parallel combination of power amplifiers has only one disadvantage, namely that when amplification in one of the un ts decreases, the other amplifiers take over its output and become overloaded. It is, therefore, necessary to test the arrangement, from time to time, and, by means of potentiometers, to adjust amplification in such a way, that the power will be evenly distributed over all the amplifiers.

In accordance with the present invention, the foregoing disadvantage is eliminated and the load is equalized among the amplifiers in a novel manner. The inputs of all the amplifiers are connected in parallel to a common source of input potential, and the outputs of all the amplifiers are connected in parallel to a common load. The individual output currents or voltages of the several amplifiers, or voltages proportional to the individual outputs, are balanced against each other, and any resulting voltage differential is applied as a corrective feedback to the input circuits of the individual amplifiers.

When all of the output voltages or currents are equal, as when the common load is shared equally by all of the amplifiers, there will be a perfect balancing of the derived individual voltages and thus no resulting feedback due to a voltage differential. However, should one amplifier have more than its share of the load, the

In one embodiment of the invention, the cut put voltage of each amplifier is compared to a voltage corresponding to its pro-rated proportion of the common load. This is effected by measuring the drop across an impedance in the common load circuit and comparing this drop with those across individual and equal impedances in'the individual amplifier outputs. The value of the load impedance is equal to the value of the individual output impedances divided by the number of amplifiers. Consequently, the voltage drop across the load impedance, with all the amplifiers feeding equally to the load, is equal to the voltage drop across each individual output impedance.

In another embodiment, each individual amplifier output includes an equally divided twosection transformer primary winding, the two sections being in phase opposition and the several windings of all the amplifiers being equal. Corresponding primary winding section of each amplifier is connected to a common line so that, upon a load unbalance, the unbalanced amplifier will have a circulating current in its primary winding section resulting in a net voltage being applied to its secondary transformer winding. This net voltage is applied as a corrective feedback to the amplifier input. In both embodiments of the invention, the corrective feedback voltage may be rectified if necessary or desirable.

With the foregoing in mind, an object of this invention is to provide an improved multiple amplifier arrangement connected between a common signal input and a common load.

Another objectis to provide such an arrangement in which equalization of the load between the individual amplifiers is effected automatically.

These, and other objects, advantages and novel features of the invention will be apparent from the following description and the accompanying drawings.

. In the drawings:

Fig. 1 is a schematic wiring diagram of a multiple amplifier arrangement embodying the invention.

Fig. 2 is a vector diagram illustrating the voltage relations.

Fig. 3 is a schematic wiring diagram of another embodiment of the invention.

Fig. 4 illustrates feedback voltage rectification incorporated in the embodiment of Fig. 3.

Referring to Fig. -1, amplifier units Z1, ZZ-Zn, have their inputs connected to a common input line so that the individual input signal voltages v are equal to the voltage e from a common preamplifier. The outputs V of the amplifiers are applied to common load lines Sbi, Sbz, leading to loudspeaker R. Impedances T1 are connected to each individual output line of the amplifier, and impedance T2 to the common load line, the impedance being dimensioned so that where n equals the number of parallel output amplifiers. Since the audio frequency current flowing from. n-amnlifiers thro gh the common impedance 1-2 is n-times larger than the currents flowing through imoedances m, but impedance-r1 is n-times larger than impedance T2, the voltage drops 61 across imoedances n will be identicalwith the Voltage drops e2 across impedance 12 as long as each of the amolifiers supplies its roper share of the Whole load. If transformers T1 and T2 ofthe same transformer ratio, e. g. l'rm are shunted across impedance T1 and 12, respectively, their secondary voltages will also be the same. This induced voltage is correctivel'y fed bacc to the amplifier'input to reduce their resultant'voltage Et=E-L1=0.

If one-of the amplifiers supplies an cutout current differing from that sup lied by the other ones, secondary vo tage E1 of transformer T1 wi l correspondingly difier from secondary voltage E2 of transformer T2, and if the secondary windings of transformers Ti and T9, are fed in the correct phase, the resultant voltage Ev will correctivelv-combine with innut voltage e0 so, that the out ut of the amplifier in ouestion will be correctively changed. The resultant voltage Ev, therefore, acts as a corrective feed-back.

Fig. 2 shows vectorially that ven when there is a phase difference between voltages E1 and'Ea th re is still a resultant corrective volta e E'v which, being fed correctively into the amplifier input, tends to remove the amnlitudeand phase diiTeren-ce between the two voltages E1 and E2.

The functioning of the other embodiment of the invention will now be explain d with r ference to Fig. 3; Allthe individual am lifiers supply current into a common line She through the primary windin s i2 of transformers T'i. This current divides in the ratio of the conductivity of windin s 34. and then unites again in the common load li e Sbc l ading to louds eaker R. If all am lifiers are evenly loaded the-currents-fiowing through windings i-2 and as are also equal. and since the number of turns of both windings is. also equal and'the windings are connect d in phase opposition, there will be no induced voltage E'v in secondary windings 55.' If. however. some one of the component amplifiers supplies into line Sbo, through windings i--2. larger curr nt than should be its r lative share. the magnetising effect of its windings I- 2 will be larger than that of its windings 34, and a voltage Ev will be induced in the secondary windings 5-5; This induc d voltage is correctively fed back to the amplifier input to reduce the larger output of this amplifier to equalize the load division. Resistors T3 in the secondary windin s 5 -8 provide an ohm c transformer load so that the corrective voltage E'v is-in phase with the amplifier current;

The advantages of this arrangement "against theone shown in Figure 1 are:

Transformer T2 is eliminated and; consequently production of this assemblyis -simpl'er;

Furthermore, it is possible permanently to build in arrangements for parallel connection of amplifiers into the component units. There is no need here for any additional arrangements, there is also no need to carry out any changes or alterations in the arrangements should the equipment be increased, no matter how many units there are connected in parallel.

Fig. 4 illustrates full wave rectifiers connected :between secondary windings 5-6 and the input feedback conductors, so that the rectified feedback voltage may be applied as a bias voltage to the input. Of" course, such rectifiers can be applied in acor'esponding manner to the ampli- 'fi'er feedbackcircuits of the arrangement of Fig 1.

While'specific embodiments of the invention have been shown and described in detail to illustrate the application of the invention principles, it should be understood that the inventiontmay be oth rwise: embodied Without departing from such principles.

I claim:

1-. A- multiple amplifier assembly comprising, in combination, a-plurality of individual amplifiers having their inputs'connected, in parallel circuit relation, to a single common source of signals and their outputs connected, in parallel circuit relation, to a single common load; individual. current measuring means each connected; inthe output of a respective amplifier, the -'current flows through each measuring means being equal. and all the output currents being balanced, when each amplifier is supplying its proper-- share of the load; and means operative, respcnsivc;to a variation in the current measuredhy anyoneof said measuring means upon its associated amplifier supplying a disproportionate-share of the load, to apply a corrective feedback potential to the input of its associated amplifier to restore the load share thereof to its proper value.

2; An assemblyas claimed in claim 1 in which said current measuring means comprises in-- dividual series impedances each connected in -a separate amplifier output, a common seriesiinpedance-connectedin seriesbetween all of the outputs and the common'load, andcircuit means operative to balance the voltage dropacrosssaid common impedance against the voltage drop across each individual impedance.

3. An assembly as claimed in claim 1 in which said: current measuring means each includes a pair of transformer primary windings'connectedin-phase oppositon, one winding of each pair measuring the individual amplifier'output C111! rent and the other winding of each pair measuring such proper share of-the load current.

4'. A multiple amplifier assembly comprising, in combination, a pluralityof individual amplifiers having their inputs connected, in parallel circuit relation, to a single common source'of signals and their outputs connected, inparallel' circuit relation; to a single common load; a plurality'of equal value individual impedances, each connected' in series inthe output of adifferent amplifier; a common impedance connected in series in the'common load circuit, the value of said common impedance being equal to that of any individual impedance divided by n, where-n equals the number of parallel connected individual amplifiers; whereby, when the load is shared equally by said amplifiers, the voltage drop "across each individual impedance will equal that across said common impedance; and means operative, responsive to a differential of said voltage drops when an amplifier supplies a disproportionate share of the common load, to apply a corrective feedback potential to the input of such amplifier to equalize its share of the load with the shares of the other amplifiers.

5. An assembly as claimed in claim 4 in which the feedback means includes rectifying means.

6. A multiple amplifier assembly comprising, in combination, a plurality of individual amplifiers having their inputs connected, in parallel circuit relation, to a single common source of signals and their outputs connected, in parallel circuit relation, to a single common load; an individual pair of transformer primary windings operatively associated with the output circuit of each amplifier, the windings of each pair being equal and connected in phase opposition; one winding of each pair being connected in series with the output circuit of its associated amplifier, the other windings of each pair carrying an equal proportionate share of the total load; and an individual secondary winding coupled to each pair REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Shallenberger Apr. 10, 1888 Potter Jan. 14, 1947 Number 

